1576
J. Chem. Eng. Data 2005, 50, 1576-1581
Interaction of the Dioxouranium(VI) Ion with Aspartate and Glutamate in NaCl(aq) at Different Ionic Strengths Antonio Gianguzza,*,† Alberto Pettignano,† and Silvio Sammartano‡ Dipartimento di Chimica Inorganica e Analitica “Stanislao Cannizzaro” dell’ Universita` di Palermo, Viale delle Scienze, I-90128 Palermo, Italy, and Dipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica dell’ Universita` di Messina, Salita Sperone 31, Messina (Villaggio S. Agata) I-98166, Italy
The formation of complexes species of the dioxouranium(VI) ion with aspartic and glutamic acids was studied in the pH range of 3 to 6 at 25 °C by potentiometric measurements (H+-glass electrode). Results gave evidence for the formation of the following species: (UO2)A0, (UO2)AH+, and (UO2)2A(OH)20 (A2- ) a glutamic or aspartic ligand). Investigations were carried out in a NaCl ionic medium at I (0.1, 0.25, 0.5, and 1.0) mol L-1. The dependence on ionic strength of the formation constants was analyzed by the specific ion interaction theory (SIT) model. The formation constants at infinite dilution, obtained using this model, are log β110 ) 8.53 ( 0.03, 8.37 ( 0.05; log β111 ) 13.60 ( 0.05, 13.42 ( 0.02; and log β21-2 ) 3.31 ( 0.05, 2.98 ( 0.03 for glutamate and aspartate, respectively, where indexes for the overall formation constant βpqr refer to the equilibrium pUO22+ + qA2- + rH+ ) (UO2)p(A)qHr(2p - 2q + r). The specific interaction coefficients are also reported. A mononuclear hydroxo species (UO2)A(OH)-, of great interest for very low naturally occurring concentrations of uranium(VI), was hypothesized, with equilibrium constant log K [(UO2)A0 + H2O ) (UO2)A(OH)-] ) -5.4 at t ) 25 °C and I ) 0 mol L-1. Speciation profiles for different concentrations of UO22+ in the presence of both amino acids are discussed.
Introduction Studies on the environmental impact and radiological effects of uranium(VI) compounds have been much improved during the last 50 years, and many articles and compilations have been published on the solution chemistry of the dioxouranium(VI) ion1-5 to define its chemical behavior in terrestrial and aquatic ecosystems. In previous papers, we reported results on the dependence on the ionic strength of hydrolysis constants of UO22+ in different ionic media6 and on the speciation of this ion in media simulating the composition of natural waters.7 Quantitative studies on the interaction of the dioxouranium ion with different carboxylic ligands were also carried out by our research group.8,9 Despite a very large amount of literature data on the solution chemistry of the uranyl ion, interactions of this ion in aqueous media with molecules of biological interest, such as amino acids, have been little investigated. Most of the results, reported many years ago, are concerned with structural investigations on the complex species formed and were carried out mainly to establish whether the interaction between uranyl and an amino acid occurs via carboxylic or amino groups or both.10-14 Quantitative studies on the stability of UO22+ amino acid complex species by different authors by means of various techniques15-23 were performed in fairly concentrated aqueous solutions of the uranyl ion (10 × 10-3 to 30 × 10-3 mol L-1) and ligand (up to CA/CM ) 10:1). Under these experimental conditions, measurements were performed generally in a very narrow pH range (2 to 4) to avoid precipitation of uranium(VI) oxide species of low solubility. Most authors report the formation of only the (UO2)A0 (A2- ) aspartic or glutamic ligand) species, often without considering the uranyl hydrolysis occurring in the pH range of 3 to 4.6,7 * Corresponding author. E-mail:
[email protected]. † Universita ` di Palermo. ‡ Universita ` di Messina.
Here we report results of a study on the interactions of UO22+ with aspartic and glutamic acids in the pH range of 2 to 6 in NaCl aqueous solution. Measurements were carried out by a potentiometric technique (H+-glass electrode) at 25 °C at fairly low concentrations of the UO22+ ion (0.5 or 1.0 mmol L-1) and with a CA/CM ratio (A2- ) aspartic or glutamic ligand; M ) UO22+) not higher than 5:1. A complex formation model was used where both the hydrolysis of the dioxouranium(VI) ion1,6,7 and the protonation of amino acids2-5,24,25 were taken into account. In addition to the simple species (UO2)A0, (A2- ) aspartic or glutamic ligand), protonated (UO2)AH+ and hydroxo (UO2)2A(OH)2° mixed species were also found. Another mononuclear hydroxo species (UO2)A(OH)-, of great interest for very low naturally occurring concentrations of uranium(VI), was hypothesized. Among the few literature data, formation constants are reported at only one ionic strength value in each work dealing with these systems, mainly 0.1 mol L-1 (NaClO4). In this work, data at different ionic strengths (0.1, 0.25, 0.5, and 1.0 mol L-1) are reported, and the specific ion interaction theory (SIT) model was used to extrapolate formation constants of all species found at infinite dilution and to calculate specific interaction parameters. Experimental Section Materials. The dioxouranium(VI) cation (UO22+) was used as the nitrate salt (Fluka), and its purity, checked gravimetrically after precipitation as U3O8 by gaseous ammonia, was >99.5 %. L(+)-aspartic and L(+)-glutamic acids (Carlo Erba, purity >99.5 %) were used without further purification. Sodium chloride was used as the anhydrous salt (Merck) after drying at 140 °C. Hydrochloric acid and sodium hydroxide solutions were prepared by diluting concentrated solutions (Aldrich and Merck, respectively) and were standardized against sodium carbon-
10.1021/je050040o CCC: $30.25 © 2005 American Chemical Society Published on Web 08/20/2005
Journal of Chemical and Engineering Data, Vol. 50, No. 5, 2005 1577 Table 1. Experimental Conditions for UO2 Glutamate and Aspartate Systems at t ) 25 °C I/mol L-1a CUO2/mol L-1 CA/mol L-1 CH/mol L-1 pH range
nb
I/mol L-1a CUO2/mol L-1 CA/mol L-1 CH/mol L-1 pH range
nb
0.0920 0.0900 0.0890 0.0910 0.0900 0.2360 0.2310 0.2310 0.2290 0.4760
0.00054 0.00054 0.00100 0.00054 0.00054 0.00052 0.00052 0.00051 0.00052 0.00054
0.00100 0.00200 0.00200 0.00150 0.00250 0.00103 0.00207 0.00152 0.00255 0.00100
0.00237 0.00475 0.00475 0.00336 0.00532 0.00287 0.00493 0.00423 0.00590 0.00235
UO2 Glutamate 3.36-6.15 76 0.4660 3.15-6.07 120 0.4610 3.13-6.07 150 0.4550 3.35-6.09 91 0.4520 3.32-6.23 121 0.9270 3.13-6.54 86 0.9250 3.13-6.53 80 0.9140 2.99-6.48 78 0.9030 3.13-6.5 89 0.8970 3.36-6.17 76
0.00054 0.00054 0.00054 0.00100 0.00054 0.00054 0.00054 0.00054 0.00100
0.00150 0.00200 0.00260 0.00200 0.00100 0.00150 0.00200 0.00260 0.00200
0.00341 0.00460 0.00552 0.00446 0.00275 0.00341 0.00446 0.00552 0.00446
3.33-6.18 3.31-6.10 3.30-5.73 3.27-5.97 3.15-6.06 3.36-6.04 3.33-6.05 3.32-5.95 3.31-6.04
92 109 126 139 89 92 109 126 137
0.0880 0.0930 0.0900 0.0940 0.0950 0.0880 0.2340 0.2280 0.2220 0.2250 0.2270
0.00057 0.00105 0.00105 0.00057 0.00057 0.00057 0.00052 0.00052 0.00091 0.00052 0.00052
0.00198 0.00099 0.00198 0.00158 0.00099 0.00396 0.00108 0.00149 0.00199 0.00202 0.00250
0.00600 0.00279 0.00477 0.00357 0.00239 0.00955 0.00295 0.00418 0.00518 0.00523 0.00580
UO2 Aspartate 2.74-6.25 128 0.4710 3.05-5.86 121 0.4660 2.90-6.06 153 0.4520 3.18-6.00 100 0.4610 3.24-6.19 79 0.4600 2.83-5.79 152 0.9190 3.04-6.44 74 0.9400 2.90-6.24 105 0.8920 2.89-6.30 95 0.9110 2.91-6.47 81 0.9200 3.00-6.41 88
0.00054 0.00054 0.00109 0.00109 0.00054 0.00054 0.00054 0.00109 0.00054 0.00054
0.00101 0.00152 0.00203 0.00101 0.00203 0.00203 0.00101 0.00203 0.00250 0.00152
0.00243 0.00345 0.00446 0.00243 0.00446 0.00447 0.00243 0.00487 0.00540 0.00386
3.23-6.09 3.18-6.16 3.11-5.99 3.21-5.91 3.14-6.04 3.16-6.07 3.25-5.88 3.02-6.01 3.15-6.02 3.05-6.02
79 95 142 110 112 112 79 154 124 109
a
Mean values of ionic strength. b Number of experimental points (two or three independent titrations).
ate and potassium hydrogen biphthalate, respectively. All of the solutions were prepared by using CO2-free freshly prepared distilled water (R ) 18 MΩ) and grade-A glassware. Experimental Equipment and Procedure. The measurements were carried out using two different potentiometric instruments consisting of a Metrohm model 665 automatic titrant dispenser coupled with a Metrohm model 654 potentiometer and combination Orion-Ross 8102 and 8172 glass electrodes, respectively. The estimated accuracy of this system was (0.15 mV and ( 0.003 mL for emf and the titrant volume reading, respectively. In titration cells, pure nitrogen was bubbled through the solutions to avoid O2 and CO2 inside, and the solutions were magnetically stirred. A volume of (20 to 25) mL of solution, containing the amino acid under investigation (glutamic or aspartic acid) and UO2(NO3)2 in different concentration ratios, in the presence of an appropriate amount of NaCl to adjust the ionic strength to different values (0.1, 0.25, 0.5, and 1.0 mol L-1) was titrated with sodium hydroxide standard solutions at (25.0 ( 0.1) °C. For each experiment, independent titrations of hydrochloric acid solutions with NaOH standard solutions were performed under the same experimental conditions of ionic strength and temperature as for the systems under study to determine the formal electrode potential; therefore, potentiometric readings were in terms of pH ) -log [H+]. All titrations were carried out up to pH ∼6 before the precipitation of uranium(VI) oxide species of low solubility. Table 1 shows the experimental conditions used for each titration carried out on the UO2 glutamate and UO2 aspartate systems. In this Table, CH refers to the total [H+] concentration derived from both the amino acids under study and a slight excess of standard HCl added to check the internal formal electrode potential and to compare it with the external one. Calculations. The nonlinear least-squares computer program ESAB2M26 was used to calculate all of the parameters of an acid-base titration (analytical concentration of the reagents, electrode potential, junction potential coefficient, and ionic product of water). The following additional computer programs27 were also used: STACO and BSTAC to calculate formation constants, LIANA for
fitting linear and nonlinear equations for the calculation of protonation and formation constants, and ES4ECI to draw distribution diagrams of the species in solution and to compute species formation. In all calculations, both the hydrolysis constants of the uranyl ion1,6,7 and the protonation constants of aspartic and glutamic acids5,24,25 in NaCl aqueous medium were taken into account. All formation constants (βpqr) are expressed according to the equilibrium
pUO22+ + qA2- + rH+ ) (UO2)p(A)qHr(2p - 2q + r) (1) Ionic Strength Dependence of Formation Constants. The dependence on ionic strength was taken into account by using the SIT model,28 according to which formation constants can be expressed as
log Km ) log TKm - z*DH + L(, I)
(2)
where Km is the molal formation constant; TKm is the relative value at infinite dilution; z* is the difference of the square of the charges; DH is the Debye-Huckel term [0.51I0.5/(1+1.5I0.5)]; and L(, I) is a linear term dependent on ionic strength and interaction coefficients. In the base theory, for a simple equilibrium
Mz+ + Az- ) MAy(in the supporting electrolyte BX), the linear term assumes the form
L(, I) ) I[((Mz+, X-) + (Az-, B+) - (MAy-, B+)] The interaction coefficients of uncharged species with ions of the supporting electrolyte are equal to zero, but the activity coefficient can be obtained from the well-known relationship28
log γ ) λI and therefore (uncharged species, supporting electrolyte) ≡ λ. Often, when some interaction coefficients are not known, eq 2 is written as
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Journal of Chemical and Engineering Data, Vol. 50, No. 5, 2005
log Km ) log TKm - z*DH + I∆
(2a)
In other words,
Table 2. Hydrolysis Constantsa,b of the Dioxouranium(VI) Ion at Different Ionic Strengths in NaCl(aq) at t ) 25 °C6 I/mol kg-1
-log β11
-log β22
-log β34
-log β35
-log β37
0.10 0.25 0.50 1.02
5.45 5.57 5.71 5.95
5.98 6.06 6.13 6.23
12.22 12.32 12.37 12.39
16.54 16.75 16.91 17.10
29.67 29.75 29.76 29.75
L(, I) ) I∆ Results and Discussion Hydrolysis Constants of the Dioxouranium(VI) Ion and Protonation Constants of Aspartic and Glutamic Acids. The dioxouranium(VI) ion undergoes strong hydrolysis with the formation of stable soluble hydroxo species (at pH >3) before the precipitation of oxide species of low solubility at pH ∼6. Most of the hydrolysis products are polynuclear species even for quite low UO22+ concentration.1 Therefore, the hydrolysis of the uranyl ion cannot be neglected when the interactions of UO22+ with organic and/or inorganic ligands in aqueous solution have to be investigated. In previous papers,6,7 we reported hydrolysis constants of UO22+ and parameters for the ionic strength dependence determined in different aqueous media and at different ionic strength values. The hydrolysis constants of the uranyl ion at the ionic strength (NaCl medium) investigated in this work are reported in Table 2. Protonation of aspartic and glutamic acids was also considered in the calculations to determine formation constants of complex species in the UO2 amino acid systems. Protonation constants of aspartic and glutamic acids at different ionic strengths in NaCl were reported in previous papers24,25 and are shown in Table 3. Formation and Stability of UO22+ Aspartate and Glutamate Complexes. Least-squares analysis of potentiometric data was performed by taking into consideration many speciation models. We found that the best set of the species is represented, for both systems, by (UO2)A0, (UO2)AH+, and (UO2)2A(OH)2° (A2- ) aspartate, glutamate). In Table 4 we report, for some experimental data, the comparison between different speciation models carried out on the basis of the residual variance of models. Formation constants of UO2 glutamate/aspartate complex species were determined at different ionic strengths in NaCl medium, and the experimental values, on the molal and molar concentration scales, are shown in Tables 5 and 6, respectively. Errors in formation constants are given as g95 % confidence intervals; these were obtained by performing separate calculations on four sets of experimental data. This procedure also partially accounts for systematic errors. The dependence of formation constants on ionic strength was taken into account by the SIT model as described in the Experimental Section. To this end, we needed to know several interaction coefficients, , for the different species. In Table 7, we report the values of (A2-, Na+), (HA-, Na+), and (H3A+,Cl-) calculated from the dependence of protonation constants on ionic strength (assuming λH2A ) 0.1, on the basis of several values reported for similar systems28) and (H+, Cl-) and (UO22+, Cl-) obtained from the literature.8,29 Using these coefficients, we calculated values for the complex species of aspartate and glutamate by fitting formation constants at different ionic strengths to eq 2. This fitting also allows us to obtain the infinite dilution formation constants. The relative results, together with formation constants at infinite dilution, are reported in Table 8. Values of and λ are fairly low, indicating small interactions between complexes and the supporting electrolyte. The stability of dioxouranium(VI) complexes with both amino acids is quite high and indicates that in addition to the two carboxylate groups the amino group is also effective
a Equilibrium constants refer to the reaction pUO 2+ + qH O 2 2 ) (UO2)p(OH)q(2p - q) + qH+(βpq). b Molal concentration scale.
Table 3. Protonation Constantsa,b of Aspartate and Glutamate at Different Ionic Strengths in NaCl at t ) 25 °C24,25 I/mol kg-1
log βH1
log βH2
log βH3
0.10 0.25 0.50 1.00
Aspartate 9.65 9.58 9.56 9.55
13.36 13.24 13.21 13.21
15.30 15.21 15.21 15.32
0.10 0.25 0.50 1.00
Glutamate 9.64 9.56 9.55 9.55
13.86 13.73 13.72 13.72
16.07 15.97 15.99 16.09
a Protonation constants refer to the equilibrium A + nH ) AH n (A ) aspartate or glutamate; n ) 1, 2, 3). b Molal concentration scale.
Table 4. Different Models Tried for the UO2 Aspartate and Glutamate Systems at I ) 0.1 mol L-1 UO2 aspartate system mean dev. b
species
log β
σ02/σ2a
MA MAH M2A(OH)2
7.588 12.634 1.984
1.000
Model 1 0.405
MA MAH
7.823 12.455
1.552
Model 2 0.973
7.85
2.654
Model 3 2.848
MA a
UO2 glutamate system log β
σ02/σ2a
mean dev.b
7.745 12.740 2.218
1.000
0.499
7.977 12.500
1.371
0.940
8.055
2.172
2.359
σ02 variance of the best model. b Mean deviation in emf.
in the coordination. In fact, at I ) 0.1 mol L-1 in NaCl we have log β110 ) 5.71 and 7.58 for malonate9 and aspartate UO2 systems, respectively. Moreover, for the analogous complex species of glycine it was found at I ) 0.1 mol L-1 (P. G. Daniele, personal communication) that log β110 ) 6.59. This means an increment in stability of 1.9 and 1 log units for the aspartate system in comparison with malonate and glycinate, respectively. Analogous considerations can be made for the protonated species (UO2)AH+ by writing the partial equilibrium
UO22+ + HA- ) (UO2)AH+
(K111)
By considering this equilibrium, we have (for the above experimental conditions) log K111 ) 3.0, 5.4, and 1.2 for aspartate, malonate, and glycinate,9 respectively, with the aspartate value being very close to the average value of glycinate and malonate [(log K111(gly) + log K111(mal))/2 ) 3.3). For the hydroxo species, the following equilibrium can be taken into account:
(UO2)2(OH)22+ + A2- ) (UO2)2A(OH)20
(K21-2)
Journal of Chemical and Engineering Data, Vol. 50, No. 5, 2005 1579 Table 5. Formation Constantsa of UO2 Glutamate Complex Species at Different Ionic Strengthsb and t ) 25 °C I/mol kg-1
log β110c
log β111
log β21-2
I/mol L-1
log β110
log β111
log β21-2
0.092 0.232 0.465 0.935
7.75 ( 7.44 ( 0.06 7.72 ( 0.04 7.63 ( 0.03
12.75 ( 0.03 12.64 ( 0.03 12.65 ( 0.03 12.66 ( 0.03
2.24 ( 0.03 2.06 ( 0.03 2.03 ( 0.04 1.95 ( 0.04
0.092 0.230 0.460 0.917
7.75 7.44 7.72 7.64
12.75 12.65 12.66 12.68
2.24 2.06 2.03 1.95
a
0.03d
Molal and molar concentration scales. b NaCl ionic medium. c log βpqr values refer to equilibrium 1.
d
g95 % confidence interval.
Table 6. Formation constantsa of UO2 aspartate complex species at different ionic strengthsb and t ) 25 °C I/mol kg-1
log β110c
log β111
log β21-2
I/mol L-1
log β110
log β111
log β21-2
0.092 0.232 0.465 0.935
7.59 ( 7.22 ( 0.05 7.37 ( 0.02 7.30 ( 0.03
12.64 ( 0.02 12.44 ( 0.02 12.43 ( 0.02 12.50 ( 0.01
1.99 ( 0.02 1.72 ( 0.03 1.72 ( 0.02 1.73 ( 0.03
0.092 0.230 0.460 0.917
7.59 7.22 7.37 7.31
12.64 12.45 12.44 12.52
1.99 1.72 1.72 1.73
a
0.02d
Molal and molar concentration scales. b NaCl ionic medium. c log βpqr values refer to equilibrium 1.
Table 7. SIT Interaction Coefficients SIT coefficients
a
glutamic acid
aspartic acid
(A2-, Na+) (HA-, Na+) λH2A (H3A+,Cl-)
0.42 0.16 0.1 0.04
0.38 0.16 0.1 0.05
(H+, Cl-)a (UO22+, Cl-)b
0.12 0.25
Reference 29. b Reference 8.
Table 8. Formation Constants at Infinite Dilution and Interaction Coefficients of UO2 Glutamate and Aspartate Complex Species in NaCl at t ) 25 °C log β110 log β111 log β21-2 λ(UO2)2A(OH)2 λUO2A (UO2HA+, Cl-) ∆110 ∆111 ∆21-2
glutamate
aspartate
8.53 ( 0.03 13.60 ( 0.05 3.31 ( 0.05 -0.046 -0.113 0.049 0.78 0.74 0.73
8.37 ( 0.05 13.42 ( 0.02 2.98 ( 0.03 -0.188 0.043 0.010 0.59 0.74 0.83
For this equilibrium, we have log K21-2 ) 8.0 and 7.0 for aspartate and malonate, respectively. These considerations support the involvement of all of the potential ligand groups in the uranyl aspartate system. Complex species in the uranyl glutamate system show a slightly higher stability that is likely due to a better steric arrangement. It is probable that other species can be formed under other experimental conditions. In particular, it is well known that at very low metal concentrations the formation of mononuclear species prevails, but the possibility of having data at very low reactant concentrations is of fundamental importance in the speciation studies of natural fluids. Therefore, for the systems here studied it should be of great interest to have an estimate of the stability of the mononuclear hydroxo species (UO2)A(OH)-, and to this end, we may resort to empirical predictive methods. For several uranyl complexes, it was shown that formation constants are a linear combination of the species stoichiometric coefficients,9 and for the reaction (UO2)A0 + H2O ) (UO2)A(OH)-, a value of formation constant log K ) -5.4 (at t ) 25 °C and I ) 0 mol L-1) was found. This means that we may predict the formation constant of the species (UO2)A(OH)- to be log β11-1 ) 3.1 and 3.0 (I ) 0 mol L-1) for glutamate and aspartate, respectively (logβ11-1 ) 2.2 and 2.1 at I ) 0.5 mol L-1). The confidence interval for these estimates (95 % confidence interval) was predicted to be (0.4 log units.
d
g95 % confidence interval.
Table 9. Literature Data of the UO2 Aspartate and UO2 Glutamate Systems ionic medium
I/mol L-1
t/°C
log β110
log K111a
NaClO4 NaClO4 NaClO4 KNO3 KCl
1 0.1 0.5 0.5 0.1 0.1 0.1 0.2 0.1
UO2 Aspartate System 25 2.41 30 8.71 25 8.40 25 8.62 30 8.34 40 8.93 50 10.40 25 2.61 30 8.00
NaClO4 KNO3 KNO3
0.1 0.1 0.2
UO2 Glutamate System 30 8.43 25 8.25 25 2.66
NaClO4 NaClO4
log K122b
ref
4.14
17 20 21 21 22 22 22 13 15
14.75
20 23 13
a The equilibrium constant refers to the reaction UO 2+ + HA2 ) (UO2)AH+. b The equilibrium constant refers to the reaction UO22+ + 2HA- ) (UO2)(AH)20.
Literature data on the formation of UO2 glutamate and aspartate complexes are reported in Table 9, where one can see that only mononuclear species were considered. As concerns the species (UO2)A0 and (UO2)AH+, the stability is comparable with that found in this work but with slightly higher formation constants. Literature data are given mostly in perchlorate and nitrate media, and this explains the significantly higher values of stability constants with respect to present data in the chloride medium. (See also, in ref 6, some consideration of the interaction of Cl- with UO22+). Some authors report, for both amino acids, formation species having stoichiometric coefficients of the ligand of >1; these species can be formed in excess ligand for protonated species such as (UO2)(AH)2° (acidic range) or at pH values close to neutrality where the formation of hydroxo complexes is highly probable. The stability of uranyl glutamate and aspartate complexes can also be compared with that of analogous complexes of other metal ions. Some formation constants for Cu2+, Ni2+, Cd2+, and Co2+ glutamate and aspartate complexes are reported in Table 10, where the stability trend Cu2+ > UO22+ > Ni2+ > Cd2+ < Co2+, in accordance with the stability of analogous series,2-5 can be observed. Because the main purpose of this work is to contribute to the speciation studies of dioxouranium(VI), we must underline that the complexes of amino acids are generally strongly dependent on temperature. In fact, in a study at different temperatures Triverdi et al.22 found (for the aspartate complex) log β110 ) 8.34, 8.93, and 10.40 at (30,
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Table 10. Literature Stability Constants2-5 of Some Metal Glutamate and Aspartate Systems at t ) 25 °C and I ) 0.1 mol L-1 metal ion
log β110
Cu2+ Ni2+ Cd2+ Co2+
M Glutamate Species 8.32 5.61 3.8 4.56
Cu2+ Ni2+ Cd2+ Co2+
M Aspartate Species 8.89 7.16 4.35 5.95
log K111a 2.89
2.92 1.52
a The equilibrium constant refers to the reaction M2+ + HA- ) MAH+.
Figure 2. Simulation of distribution of species vs pH in the systems (a) H+-Na+-UO22+-asp2--Cl- and (b) H+-Na+-UO22+glu2--Cl- at t ) 25 °C. Species: 1, (UO2)(OH)-; 2, (UO2)2(OH)22+; 3, (UO2)3(OH)42+; 4, (UO2)3(OH)5+; 5, (UO2)3(OH)7-; 6, (UO2)AH+; 7, (UO2)A0; 8, (UO2)2A(OH)2°; 9, (UO2)A(OH)-.
Figure 1. Distribution of species vs pH in the systems (a) H+-Na+-UO22+-asp2--Cl- and (b) H+-Na+-UO22+-glu2--Clat t ) 25 °C. Species: 1, (UO2)(OH)+; 2, (UO2)2(OH)22+; 3, (UO2)3(OH)42+; 4, (UO2)3(OH)5+; 5, (UO2)3(OH)7-; 6, (UO2)AH+; 7, (UO2)A0; 8, (UO2)2A(OH)2°.
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Received for review January 26, 2005. Accepted May 17, 2005. We thank the Universities of Palermo and Messina for financial support (PRA).
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